1. Technical Field
The disclosure relates to a transformer incorporated in electronic circuits such as DC-DC converters.
2. Related Art
Some DC-DC converters use transformers to perform voltage conversion of DC power. One of such DC-DC converters is shown in FIGS. 1A, 1B and 2. FIG. 1A is a plan view illustrating a transformer based on conventional art. FIG. 1B is a cross-sectional view taken along a line A-A of FIG. 1A. FIG. 2 is an explanatory view illustrating vibration of the transformer based on conventional art.
As shown in FIG. 1B, a transformer 9 is fixed to a base plate 6 that is a metal plate made of aluminum or the like. The transformer 9 includes a lower core 2, at least two upper cores 3, primary coils 41 and a secondary coil 42. The lower core 2 is made of a magnetic material and arranged on the base plate 6. The two upper cores 3 are arranged face to face over the upper surface of the lower core 2. The primary coils 41 and the secondary coil 42 are arranged between the lower core 2 and the upper cores 3 (e.g., see JP-A-2005-051995).
Each upper core 3 is in contact with the lower core 2 on the outer side of the primary coils 41 and the secondary coil 42. Also, a first gap 11 is formed between each upper core 3 and the lower core 2, on the inner side of the primary coils 41 and the secondary coil 42. Further, the two upper cores 3 are extended towards each other, i.e. extended from the outer side of the primary coils 41 and the secondary coil 42 toward the inner side of these coils, with a second gap 12 being provided between opposing surfaces of the upper cores 3.
Thus, a magnetic path that passes the inner side and the outer side of the primary coils 41 and the secondary coil 42 is formed by the lower core 2 and the upper cores 3, while the occurrence of magnetic saturation is prevented by the first gaps 11.
However, in the transformer 9, ripple current is caused due to the presence of the first gap 11. The ripple current may pass through the primary coils 41, as shown in FIG. 2, and cause fluctuations in the magnetic flux φ. In such a case, a magnetic attractive force F is generated in the first gap 11, by which the lower core 2 and the upper core 3 are attracted to each other, and at the same time, the magnitude of the magnetic attractive force F is varied. Accordingly, in each first gap 11, the upper core 3 and the lower core 2 vibrate such that these cores 3 and 2 mutually come closer and are mutually drawn apart (see the arrow V of FIG. 2), causing noise (vibration noise). In other words, the vibration of the cores 3 and 2 is transmitted to the vehicle cabin, for example, of the vehicle that installs the DC-DC converter, and generates noise.